1. Field of the Invention
This invention relates to polyester compositions suitable for injection molding. More particularly, the invention relates to compositions based on linear and saturated polyesters which, after addition of nucleating and plasticizing agents, make it possible to obtain high crystallization rates even when operating with water-heated molds.
2. Discussion of the Prior Art
Saturated linear thermoplastic polyesters such as PET, its blends and copolymers, are produced on a wide scale at comparatively low cost and they show, in their crystalline state, excellent physical properties consistently with a high melting point.
The formulations based on the aforesaid polyesters, however, can be scarcely applied in the field of injection molding, mainly because of the difficulties in obtaining conveniently crystallized articles when using the usual molds. A non-modified PET crystallizes very slowly in the melted state, making the process uneconomical and producing articles having a crystalline structure of non-homogeneous spherolitic type, low impact strength, and a rough surface.
The processing of said PET requires high molding temperatures, and long residence times in the mold are required in order to obtain dimensionally stable pieces. In fact, an early extraction of a piece from the mold, not sufficiently crystallized, can involve a further crystallization with time, with subsequent change in the volume and possible distortion of the shape.
From a practical point of view, it is desirable to obtain satisfactory properties as to the crystallization rate by resorting to the usual water-heated molds.
Additives known to the prior art make it possible profitably to increase the crystallization rate in the melted state of the polyesters. These additives, however, affect only in a secondary way the temperature of cold crystallization, a parameter determining the lowest limit of the molding temperature. In fact, in the case of non-modified PET, the cold crystallization peak, measured by a differential calorimeter, presents its maximum at the temperature of 135.degree. C and when nucleating agents of the saline type (sodium benzoate, sodium o-chlorobenzoate and the like) are added to said PET, such temperature is lowered only a little (130.degree. C).
Examples of such formulations are described, for instance, in the following patents or patent applications: U.S. Pat. No. 3,516,957; JP-71/029,977; DE-2,014,770; DE-1,945,967; and DE-1,945,101.
It is known that in order to influence significantly the cold crystallization temperature of a resin, as well as the glass transition temperature of the amorphous phase, it is necessary to use plasticizers.
The patent literature teems, in this connection, with references citing the use of the usual plasticizers in combination with PET. It is generally a question of aliphatic esters of phthalic, adipic or sebacic acids or of aliphatic and aromatic esters of phosphoric acid.
Such compositions are described for instances in the following patents or patent applications: U.S. Pat. No. 4,344,874; U.S. Pat. No. 4,440,889; and JP-83/049747.
These classes of plasticizers, particularly developed for PVC, do not show, however, sufficient interaction with the polyester resin both as to the miscibility (the formation of separate phases is observed) and as to the effectiveness (scanty influence on the glass transition temperature (Tg), on the viscosity of the melted product, on the modulus, and so on). Only a few aromatic esters, having a low molecular weight, and the polyoxyalkyleneglycols show some degree of interaction with the resin. Among the first compounds neopentyl glycol dibenzoate is generally the most frequently used, but the use is also described of diethylene glycol dibenzoate, triethylene glycol dibenzoate, glyceryl tribenzoate, and the like. Examples of such compositions are described, among others in U.S. Pat. No. 4,440,889 and U.S. Pat. No. 4,344,874. This class of products, owing to their low boiling point, gives rise during the processing, however, to plentiful as well as undesired formation of smokes. More interesting from this point of view is the use of polyethers of the polyoxyalkylene-glycol type.
The patent literature teems with references relating to polyester compositions containing a nucleating agent and a plasticizing agent of the polyoxyalkylene-glycol type.
For instance, JP-A-84/157,144 describes the nucleation and plasticization of PET by means of Na stearate and of polyoxyethylene glycol-lauryl-glycidyl-ether. JP-A-84/24,747, on the contrary, describes the use of a copolymer ethylenemethacrylic acid, salified with Na in combination with polyoxyethylene glycol dimethyl ether, in order to improve the molding of PET, JP-A-57/143,384 describes a polyester composition modified by the addition of polyoxyethylene glycol diphthalate disodium salt. U.S. Pat. No. 4,365,036 describes a nucleating agent based on a complex between NaI and polyoxyethylene glycol. U.S. Pat. No. 4,558,085, on the contrary, describes a nucleant based on a complex between polyoxyethylene glycol and sodium salts of aliphatic acids. However, by using these additives no satisfactory result can be achieved, especially as far as molding is concerned.